Phosphoric acid-glycerine electrolyte for hygroscopic cell



g 22, 1967 P. GOLDSMITH 3,337,441

PHOSPHORIC ACIDGLYCERINE ELECTROLYTE FOR HYGROSCOPIC CELL Filed Aug. 7. 1964 United States Patent Ofilice 3,337,441 Patented Aug. 22, 1967 3,337,441 PHOSPHORIC ACID-GLYCERINE ELECTROLYTE FOR HYGROSCOPIC CELL Philip Goldsmith, Brightwell-cum-Sotwell, near Wallingford, England, assignor to United Kingdom Atomic Energy Authority, London, England Filed Aug. 7, 1964, Ser. No. 388,198 Claims priority, application Great Britain, Aug. 19, 1963, 32,720/ 63 13 Claims. (Cl. 204- 195) This invention relates to apparatus for the determination of the humidity of gases and is particularly concerned with an improved form of electrolytic cell for use in such apparatus.

It has been previously proposed that moisture sensitive electrolytic cells should include a pair of spaced electrodes having a continuous coating of a suitable hygroscopic electrolyte formed over and between the electrodes to absorb moisture in a gas flowing through the cell. Phosphorous pentoxide has been suggested as the electrolyte but cells so constructed tend to have a slow response time in relation to changes in moisture content in the gas.

The object of the present invention is to provide an I improved electrolytic cell in which the time of response of the cell, to changes in moisture content of a gas flowing through the cell, is minimised.

According to the invention an electrolytic cell for the measurement of the water vapour content of a gas comprises a pair of electrodes spaced apart on a support member and a hygroscopic electrolytic film extending over the electrodes and support member, said film comprising a mixture of phosphoric acid and glycerine.

Preferably the electrolytic film includes substantially equal amounts by volume of phosphoric acid and glycerine, the electrolyte being applied to the electrodes and support member as a solution of phosphoric acid, glyc crime and distilled water.

The composition of the solution preferably lies within the range 4 to 10 parts by volume each of phosphoric acid and glycerine per 100 parts distilled Water.

According to one feature of the invention the solution comprises parts phosphoric acid, 5 parts glycerine and 100 parts distilled water.

According to further features of the invention an electrolytic cell for the measurement of the water vapour content of a gas comprises a tubular case, an inner cylindrical member coaxial with said case and spaced therefrom to define an annular gas passage, a double start helical thread on said member, electrode located in each thread groove and a film of hygroscopic electrolyte covering the electrodes, said film comprising equal amounts by volume of glycerine and phosphoric acid.

Preferably the inner member is provided with a short axial bore at each end, each bore communicating with the annular gas space and an electrode is threaded through a bore into a thread groove from each end of the member.

To enable the nature of the invention to be more readily understood one embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing. In the drawing:

FIG. 1 is a pictorial view of an electrolytic cell.

FIG. 2 is a fragmentary pictorial view showing the central portion of the cell of FIG. 1 to a large scale.

Referring to the drawing the electrolytic cell comprises a tubular stainless steel case 1, a cylindrical inner memher 2 coaxial with the case and spaced therefrom to provide an annular passage 3 for the gas being tested. The member 2, formed from silica, has a double start helical thread formed over part of its length to locate electrodes 4 and 5. The electrodes are covered by a layer of hygroscopic electrolyte comprising a mixture of phosphoric acid and glycerine.

In the embodiment described the case 1 is .7 cm. in ternal diameter, the central member is .6 cm. diameter and 9 cm. long and is provided with a shoulder portion .7 cm. diameter and 1.25 cm. long at the end to provide the annular gas passage. Inlet and outlet ducts 6 and 7 respectively complete the gas flow passage through the cell.

The double start helical thread has a pitch of 40 threads per cm. and the electrodes 4 and 5 are formed from platinum wire .05 mm. in diameter providing two electrodes approximately .075 mm. apart and 400 cm. long.

The central member 2 is provided with a short axial bore 8 at each end, each bore communicates with the annular gas space 3 through a duct 9 arranged normal to the axis of the member. An end of an electrode is anchored to the member at 10 using a suitable adhesive i.e. Araldite and the electrode is wound along a thread groove and subsequently passed through a duct 9 and bore 8 to the outside of the cell. For example, one end of electrode 4 is anchored at the right hand end of the central member, the electrode is wound into and located in a thread groove and the other end passed through the duct 9 and bore 8 at the left hand end of the member, While electrode 5 is anchored to the left hand end and exits through the bore at the right hand end of the central member. Each electrode is sealed, again using an adhesive, into the duct 9 and at the exit points on the axial bores 8.

During construction the unwound silica member is cleaned and degreased using sulphuric acid-chromic acid mixtures and solvents and subsequently washed in distilled water. After winding the electrodes into the thread grooves the degreasing and washing steps are repeated. The central member is sensitized by dripping a solution of 5 parts phosphoric acid, 5 parts glycerine and parts distilled water by volume on to the electrode windings, the member being rotated until an even and continuous film of phosphoric acid covers the windings. It will be appreciated that the film on the electrodes upon partial drying will comprise phosphorus pentoxide, phosphoric acid and glycerine. By phosphoric acid we mean the common phosphoric acid sometimes referred to as orthophosphoric acid and obtained by dissolving phosphorus pentoxide in water and boiling to convert the resultant metaphosphoric acid into orthophosphoric acid. The wound silica member is then sealed into the stainless steel tubular case using black vacuum wax or a P.T.F.E. sealing washer.

The addition of about 5 parts by volume of glycerine to the phosphoric acid-water sensitising solution has proved advantageous in practice in decreasing the response time of the cell. Without this addition the response time becomes slower the drier the gas being measured. Such addition has reduced the response time for the instrument described to one minute for air containing only 1 part per million of water and to 20-30 seconds for 2 parts per million of water. The addition of glycerine makes the absorbent film less viscous at low humidities thereby increasing the mobility of the ions during electrolysis and increasing the response time of the cell.

The composition of the sensitizing solution may be varied within the range 10 parts each of phosphoric acid and glycerine per 100 parts distilled water without difficulty but the preferred composition in 5 parts each of phosphoric acid and glycerine per 100 parts distilled water.

In operation air or other gases are passed through the annular space 3 at such a flow rate that all the water vapour molecules in the gas can difiuse on to the sensitised surface. The water vapour is electrolyzed to hydrogen and oxygen and the electrolysis current in the electrode is a measure of the rate at which water is being absorbed. For the cell described a flow rate of up to 700 ml./min. at S.T.P. has proved suitable.

The voltage applied across the cell is not critical but the best response time resulted when using a potential between 72 and 120 volts.

I claim:

1. An electrolytic cell for the measurement of the Water vapour content of a gas comprises a pair of electrodes spaced apart on a support member and a hygroscopic electrolytic film extending over the electrodes and support member, said film comprising a mixture of phosphoric acid and glycerine.

2. An electrolytic cell for the measurement of the water vapour content of a gas comprises a tubular case, an inner cylindrical member coaxial with said case and spaced therefrom to define an annular gas passage, a double start helical thread on said member, an electrode located in each thread groove and a film of hygroscopic electrolyte covering the electrodes, said film comprising a mixture of phosphoric acid and glycerine.

3. An electrolytic cell according to claim 1 wherein the electrolytic film includes substantially equal amounts by volume of phosphoric acid and glycerine.

4. An electrolytic cell according to claim 3 wherein the inner cylindrical member is formed from silica and the electrodes are platinum.

5. An electrolytic cell according to claim 2 wherein the inner cylindrical member is formed from silica and the electrodes are platinum.

6. An electrolytic cell for the measurement of the water vapour content of a gas comprises a tubular case, an inner cylindrical member coaxial with said case and spaced therefrom to define an annular gas passage, a double start helical thread on said member, an electrode located in each thread groove, means defining a short axial bore at each end of the inner cylindrical member, each bore communicating with the annular gas space, one of said electrodes being threaded through a bore into a thread groove from each end of the member and a film of hygroscopic electrolyte covering the electrodes, said film comprising a mixture of phosphoric acid and glycerine.

7. A method of constructing an electrolytic cell for the measurement of the water vapour content of a gas comprising the steps of spacing a pair of electrodes apart on a support member and forming a continuous film of hydroscopic electrolyte on the electrodes and support member by coating said electrodes and support member with a solution comprising phosphoric acid and glycerine.

8. A method of constructing an electrolytic cell according to claim 6 wherein the solution includes substantially equal amounts of phosphoric acid and glycerine in distilled water.

9. A method of constructing an electrolytic cell according to claim 8 wherein the solution comprises 5 parts phosphoric acid, 5 parts glycerine and parts distilled water.

10. A method of constructing an electrolytic cell according to claim 7 wherein the composition of the solution lies within the range of 4 to 10 parts each of phosphoric acid and glycerine per 100 parts of distilled water.

11. A method of constructing an electrolytic cell according to claim 7 wherein the solution comprises 5 parts phosphoric acid, 5 parts glycerine and 100 parts distilled water.

12. An electrolytic cell according to claim 2 wherein the electrolytic film includes substantially equal amounts by volume of phosphoric acid and glycerine.

13. An electrolytic cell according to claim 12 wherein the inner cylindrical member is formed from silica and the electrodes are platinum.

References Cited UNITED STATES PATENTS 2,830,945 4/1958 Keidel 204l95 3,072,556 l/l963 Czuka 204 3,194,072 7/1965 Vaala 73-356 JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner. 

1. AN ELECTROLYTIC CELL FOR THE MEASUREMENT OF THE WATER VAPOUR CONTENT OF A GAS COMPRISES A PAIR OF ELECTRODES SPACED APART ON A SUPPORT MEMBER AND A HYGROSCOPIC ELECTROLYTIC FILM EXTENDING OVER THE ELECTRODES AND SUPPORT MEMBER, SAID FILM COMPRISING A MIXTURE OF PHOSPHORIC ACID AND GLYCERINE. 